The present invention relates to a sheet feed device for use in a printer or the like.
Heretofore, a sheet feed device for use in a printer includes a peeling roller disposed in the upper portion of a hopper in which cut sheets are stacked. The peeling roller is in frictional contact with the uppermost sheet and feeds out the latter from the hopper toward a printing mechanism of the printer. However, when it is intended to print on a number of overlapping sheets, such as no-carbon sheets, at a time, the sheets are not smoothly fed only by the peeling roller, because the upper and lower sheets tend to be displaced from each other and the sheets tend to be curved before their leading edges reach the printing mechanism. When a short-length sheet of paper, such as a postcard, is fed by the sheet feed device, the sheet is released from the peeling roller before the leading edge thereof reaches the printing mechanism. The provision of only the peeling roller is insufficient to smoothly and perfectly feed various kinds of sheets.
To avoid the above shortcoming, it has been proposed to interpose main and auxiliary feed rollers between the peeling roller and the printing mechanism. The main feed roller is forcibly rotated by a drive source, such as a motor, and the auxiliary feed roller is rotatable in compliance with the rotation of the main feed roller. More specifically, these rollers are in frictional contact with each other so that the auxiliary feed roller is rotated in accordance with the rotation of the main feed roller with a sheet of paper being gripped therebetween. Alternatively, the main and auxiliary feed rollers are provided with intermeshing gears coupled to the ends of the rollers and the motor forcibly rotates the rollers simultaneously through the gears.
In the sheet feed device having the main and auxiliary feed rollers thus arranged, the auxiliary feed roller prevents the sheet from being curved when a long-length sheet is fed. When a short-length sheet is fed, it is prevented from being released from the auxiliary feed roller before it reaches the printing mechanism. The thus arranged sheet feed device rotates the rollers at all times irrespective of the length of the sheet to be fed, thereby feeding the sheet to the printing mechanism.
However, the following shortcomings are still outstanding in the proposed sheet feed device. Firstly, both the peeling roller located in an upstream position and the main and auxiliary feed rollers in the downstream position are driven by a single motor, so that the motor undergoes a large load and has a short service life. Secondly, when only one thin sheet of paper is to be fed, the auxiliary feed roller is substantially accurately rotated by the main feed roller; however, in the case where the auxiliary feed roller is rotated through the frictional contact with the main feed roller, when a number of overlapping sheets are to be fed, the rotation of the auxiliary feed roller is largely delayed with respect to the rotation of the main feed roller due to slippage between the sheets. A speed difference is developed between upper and lower sheets, and these sheets tend to be fed in different directions. As a result, the sheets being fed may be jammed or wrinkled. Thirdly, in the case where the main and auxiliary rollers are coupled through the intermeshing gears, a number of overlapping sheets can substantially accurately be fed as there is no substantial speed difference between the rollers; however, since it is highly difficult to fabricate the rollers such that their outer circumferential surfaces are in exact agreement with the pitch circles of the gears, the peripheral speed of one of the forcibly rotated rollers is liable to be higher or lower than the peripheral speed of the other roller. Fourthly, when the rollers are rotated in direct contact with each other without any sheets gripped therebetween, the motor is subjected to a very large load because the outer circumferential surfaces of the rollers have to slip forcibly against each other. Therefore, the motor must produce a large output power. Even if the motor is powerful enough to rotate the rollers, the outer circumferential surfaces of the rollers may get peeled off. This drawback holds true for an application in which a material that is less slippery with respect to the rollers is fed by the rollers.